1. Field of the Invention
Aspects of the invention relates to onboard semiconductor devices having a voltage-controlled type semiconductor elements.
2. Description of the Related Art
A semiconductor device is provided with a voltage-controlled type semiconductor element composed of an insulated gate bipolar transistor, a power MOSFET, or the like. Japanese Unexamined Patent Application Publication No. 2006-037822 (also referred to herein as “Patent Document 1”) discloses an onboard semiconductor device used for an ignition control device for an internal combustion engine, in which an end of the primary winding of an ignition coil is connected to a battery and the other end is connected to the ground through a voltage-controlled type semiconductor element. To turn ON the voltage-controlled type semiconductor element, an external electronic control unit (ECU) delivers an input signal at a specified voltage to an input terminal of the voltage-controlled type semiconductor element 21 through a gate resistor, thereby raising the gate voltage to turn ON the voltage-controlled type semiconductor element.
On the other hand, to turn OFF the voltage-controlled type semiconductor element, the charges accumulated on the gate capacitance of the voltage-controlled type semiconductor element are discharged toward the external electronic control unit. The discharging circuit comprises a series circuit of a speed-up diode and a resistor in parallel to the gate resistor for by-passing the gate resistor in the discharging process. This circuit construction increases dl/dt of the gate current in the discharging process. The resistance value of the gate resistor is set in the range of 1 to 10 kΩ, and the resistance value of the resistor connected to the speed-up diode in series is set in the range of 50Ω to 1 kΩ.
In the conventional device disclosed in Patent Document 1, the input signal is delivered through the gate resistor in the process of turning ON the voltage-controlled type semiconductor element, and the charges in the gate capacitance are discharged in the process of turning OFF by-passing the gate resistor by the series circuit of the speed-up diode and the resistor. Thus, the two different paths are formed for the processes of turning ON and turning OFF of the voltage-controlled type semiconductor element.
This type of semiconductor device is vulnerable to high frequency noise at an order of several MHz such as radio noise. When a high frequency noise is superposed on a signal line for delivering the input signal to the voltage-controlled type semiconductor element, the gate potential of the voltage-controlled type semiconductor element oscillates in the range of 0 to about 10 V due to the high frequency noise. The oscillation of the gate potential of the voltage-controlled type semiconductor element generates oscillating charge and discharge for the gate potential. FIG. 3 illustrates the oscillation of the gate potential. The high frequency noise is assumed to have a high frequency sinusoidal waveform, as shown by the waveform (a) in FIG. 3. A positive direction of current flow is defined as the direction toward the gate of the voltage-controlled type semiconductor element, and a negative direction is defined as the direction from the gate of the voltage-controlled type semiconductor element toward the input signal supplying side.
When a harmonic wave is superposed on the delivered input signal, during the period from the starting time t20 to the time t21, shown by the waveform (c) in FIG. 3, the gate capacitance of the voltage-controlled type semiconductor element is charged through a relatively large gate resistance, changing the gate current Ig.
Then during the period from the time t21 to the time t22, the gate potential increases in the negative direction and the charges on the gate capacitance of the voltage-controlled type semiconductor element are discharged toward the input signal supplying side. In this period, however, the gate potential does not exceed the forward voltage drop Vf, which is 0.6 V, for example, of the speed-up diode, and thus the speed-up diode remains in an OFF state and the current flows through the gate resistor toward the input signal supplying side.
Then, at the time t22, the speed-up diode turns ON and the charges on the gate capacitance of the voltage-controlled type semiconductor element are discharged through the speed-up diode and a resistor having a relatively low resistance toward the input signal supplying side. As a consequence, the gate current rapidly increases in the negative direction and then decreases until the time t23 at which the speed-up diode returns into an OFF state. After that, the current flows again through the gate resistor from the gate of the voltage-controlled type semiconductor element, with a slow decreasing rate of the gate current Ig.
Then, at the time t24, the gate current from the input signal supplying side to charge the gate capacitance of the voltage-controlled type semiconductor element 21 starts to increase in the positive direction similarly to the event at the time t20.
The current running in the gate wiring has a larger amplitude of the gate current −Ig flowing during the period between t21 and t24 than the amplitude of the gate current +Ig flowing during the period between t20 and t21. Thus, when a high frequency noise is superposed on the input signal, the average value of the gate current Ig significantly decreases below zero, as indicated by the dotted characteristic line L1 in the waveform (c) in FIG. 3.
Because the average value of the gate current Ig significantly decreases below zero when a high frequency noise is superposed on the input signal, a phenomenon occurs that the average value of the gate voltage decreases below the input signal of 5V. When charging and discharging, or ON and OFF, of the gate voltage are repeated due to a high frequency noise, the speed of charging and the speed of discharging are different from each other on the gate capacitance of the voltage-controlled type semiconductor element. The apparent drop of the gate potential decreases the current that flows through the voltage-controlled type semiconductor element. Thus, a problem arises that the output in the secondary winding of the ignition coil decreases.